Apophis

back contamination
Betelgeuse: What will happen when it explodes?
black hole

Chicxulub crater
cosmic collisions, biological effects
Cretaceous-Tertiary boundary

Doomsday Machine

earthquake
Elenin, Comet
extraterrestrial civilizations, hazards to

gamma-ray burst
gray goo

ice ages

hypernova

LINEAR (Lincoln Near Earth Asteroid Research) program

mass extinction
magnetic field reversal
megacatastrophe

near-Earth asteroid
near-Earth object

potentially hazardous asteroid

Siding Spring Survey
Snowball Earth
Spaceguard Foundation
Spacewatch Project
supernova

Torino scale
tsunami
Tunguska phenomenon

volcano